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Concept

Enzymes are biological catalysts, or chemicals that speed up the rate of reaction between substances without themselves being consumed in the reaction. As such, they are vital to such bodily functions as digestion, and they make possible processes that normally could not occur except at temperatures so high they would threaten the well-being of the body. A type of protein, enzymes sometimes work in tandem with non-proteins called coenzymes. Among the processes in which enzymes play a vital role is fermentation, which takes place in the production of alcohol or the baking of bread and also plays a part in numerous other natural phenomena, such as the purification of wastewater.

How It Works

Amino Acids, Proteins, and Biochemistry

Amino acids are organic compounds made of carbon, hydrogen, oxygen, nitrogen, and (in some cases) sulfur bonded in characteristic formations. Strings of 50 or more amino acids are known as proteins, large molecules that serve the functions of promoting normal growth, repairing damaged tissue, contributing to the body's immune system, and making enzymes. The latter are a type of protein that functions as a catalyst, a substance that speeds up a chemical reaction without participating in it. Catalysts, of which enzymes in the bodies of plants and animals are a good example, thus are not consumed in the reaction.

Catalysts

In a chemical reaction, substances known as reactants interact with one another to create new substances, called products. Energy is an important component in the chemical reaction, because a certain threshold, termed the activation energy, must be crossed before a reaction can occur. To increase the rate at which a reaction takes place and to hasten the crossing of the activation energy threshold, it is necessary to do one of three things.

The first two options are to increase either the concentration of reactants or the temperature at which the reaction takes place. It is not always feasible or desirable, however, to do either of these things. Many of the processes that take place in the human body, for instance, normally would require high temperatures—temperatures, in fact, that are too high to sustain human life. Imagine what would happen if the only way we had of digesting starch was to heat it to the boiling point inside our stomachs! Fortunately, there is a third option: the introduction of a catalyst, a substance that speeds up a reaction without participating in it either as a reactant or as a product. Catalysts thus are not consumed in the reaction. Enzymes, which facilitate the necessary reactions in our bodies without raising temperatures or increasing the concentrations of substances, are a prime example of a chemical catalyst.

The Discovery of Catalysis

Long before chemists recognized the existence of catalysts, ordinary people had been using the chemical process known as catalysis for numerous purposes: making soap, fermenting wine to create vinegar, or leavening bread, for instance. Early in the nineteenth century, chemists began to take note of this phenomenon. In 1812 the Russian chemist Gottlieb Kirchhoff (1764-1833) was studying the conversion of starches to sugar in the presence of strong acids when he noticed something interesting.

When a suspension of starch (that is, particles of starch suspended in water) was boiled, Kirchhoff observed, no change occurred in the starch. When he added a few drops of concentrated acid before boiling the suspension, however, he obtained a very different result. This time, the starch broke down to form glucose, a simple sugar (see Carbohydrates), whereas the acid—which clearly had facilitated the reaction—underwent no change. In 1835 the Swedish chemist Jöns Berzelius (1779-1848) provided a name to the process Kirchhoff had observed: catalysis, derived from the Greek words kata ("down") and lyein ("loosen"). Just two years earlier, in 1833, the French physiologist Anselme Payen (1795-1871) had isolated a material from malt that accelerated the conversion of starch to sugar, for instance, in the brewing of beer.

The renowned French chemist Louis Pasteur (1822-1895), who was right about so many things, called these catalysts ferments and pronounced them separate organisms. In 1897, however, the German biochemist Eduard Buchner (1860-1917) isolated the catalysts that bring about the fermentation of alcohol and determined that they were chemical substances, not organisms. By that time, the German physiologist Willy Kahne had suggested the name enzyme for these catalysts in living systems.

Substrates and Active Sites

Each type of enzyme is geared to interact chemically with only one particular substance or type of substance, termed a substrate. The two parts fit together, according to a widely accepted theory introduced in the 1890s by the German chemist Emil Fischer (1852-1919), as a key fits into a lock. Each type of enzyme has a specific three-dimensional shape that enables it to fit with the substrate, which has a complementary shape.

The link between enzymes and substrates is so strong that enzymes often are named after the substrate involved, simply by adding ase to the name of the substrate. For example, lactase is the enzyme that catalyzes the digestion of lactose, or milk sugar, and urease catalyzes the chemical breakdown of urea, a substance in urine. Enzymes bind their reactants or substrates at special folds and clefts, named active sites, in the structure of the substrate. Because numerous interactions are required in their work of catalysis, enzymes must have many active sites, and therefore they are very large, having atomic mass figures as high as one million amu. (An atomic mass unit, or amu, is approximately equal to the mass of a proton, a positively charged particle in the nucleus of an atom.)

Suppose a substrate molecule, such as a starch, needs to be broken apart for the purposes of digestion in a living body. The energy needed to break apart the substrate is quite large, larger than is available in the body. An enzyme with the correct molecular shape arrives on the scene and attaches itself to the substrate molecule, forming a chemical bond within it. The formation of these bonds causes the breaking apart of other bonds within the substrate molecule, after which the enzyme, its work finished, moves on to another uncatalyzed substrate molecule.

Coenzymes

All enzymes belong to the protein family, but many of them are unable to participate in a catalytic reaction until they link with a non protein component called a coenzyme. This can be a medium-size molecule called a prosthetic group, or it can be a metal ion (an atom with a net electric charge), in which case it is known as a cofactor. Quite often, though, coenzymes are composed wholly or partly of vitamins. Although some enzymes are attached very tightly to their coenzymes, others can be parted easily; in either case, the parting almost always deactivates both partners.

The first coenzyme was discovered by the English biochemist Sir Arthur Harden (1865-1940) around the turn of the nineteenth century. Inspired by Buchner, who in 1897 had detected an active enzyme in yeast juice that he had named zymase, Harden used an extract of yeast in most of his studies. He soon discovered that even after boiling, which presumably destroyed the enzymes in yeast, such deactivated yeast could be reactivated. This finding led Harden to the realization that a yeast enzyme apparently consists of two parts: a large, molecular portion that could not survive boiling and was almost certainly a protein and a smaller portion that had survived and was probably not a protein. Harden, who later shared the 1929 Nobel Prize in chemistry for this research, termed the non protein a coferment, but others began calling it a coenzyme.

Real-Life Applications

The Body, Food, and Digestion

Enzymes enable the many chemical reactions that are taking place at any second inside the body of a plant or animal. One example of an enzyme is cytochrome, which aids the respiratory system by catalyzing the combination of oxygen with hydrogen within the cells. Other enzymes facilitate the conversion of food to energy and make possible a variety of other necessary biological functions. Enzymes in the human body fulfill one of three basic functions. The largest of all enzyme types, sometimes called metabolic enzymes, assist in a wide range of basic bodily processes, from breathing to thinking. Some such enzymes are devoted to maintaining the immune system, which protects us against disease, and others are involved in controlling the effects of toxins, such as tobacco smoke, converting them to forms that the body can expel more easily.

A second category of enzyme is in the diet and consists of enzymes in raw foods that aid in the process of digesting those foods. They include proteases, which implement the digestion of protein; lipases, which help in digesting lipids or fats; and amylases, which make it possible to digest carbohydrates. Such enzymes set in motion the digestive process even when food is still in the mouth. As these enzymes move with the food into the upper portion of the stomach, they continue to assist with digestion.

The third group of enzymes also is involved in digestion, but these enzymes are already in the body. The digestive glands secrete juices containing enzymes that break down nutrients chemically into smaller molecules that are more easily absorbed by the body. Amylase in the saliva begins the process of breaking down complex carbohydrates into simple sugars. While food is still in the mouth, the stomach begins producing pepsin, which, like protease, helps digest protein.

Later, when food enters the small intestine, the pancreas secretes pancreatic juice—which contains three enzymes that break down carbohydrates, fats, and proteins—into the duodenum, which is part of the small intestine. Enzymes from food wind up among the nutrients circulated to the body through plasma, a watery liquid in which red blood cells are suspended. These enzymes in the blood assist the body in everything from growth to protection against infection.

One digestive enzyme that should be in the body, but is not always present, is lactase. As we noted earlier, lactase works on lactose, the principal carbohydrate in milk, to implement its digestion. If a person lacks this enzyme, consuming dairy products may cause diarrhea, bloating, and cramping. Such a person is said to be "lactose intolerant," and if he or she is to consume dairy products at all, they must be in forms that contain lactase. For this reason, Lactaid milk is sold in the specialty dairy section of major supermarkets, while many health-food stores sell lactaid tablets.

Fermentation

Fermentation, in its broadest sense, is a process involving enzymes in which a compound rich in energy is broken down into simpler substances. It also is sometimes identified as a process in which large organic molecules (those containing hydrogen and carbon) are broken down into simpler molecules as the result of the action of microorganisms working anaerobically, or in the absence of oxygen. The most familiar type of fermentation is the conversion of sugars and starches to alcohol by enzymes in yeast. To distinguish this reaction from other kinds of fermentation, the process is sometimes termed alcoholic or ethanolic fermentation.

At some point in human prehistory, humans discovered that foods spoil, or go bad. Yet at the dawn of history—that is, in ancient Sumer and Egypt—people found that sometimes the "spoilage" (that is, fermentation) of products could have beneficial results. Hence the fermentation of fruit juices, for example, resulted in the formation of primitive forms of wine. Over the centuries that followed, people learned how to make both alcoholic beverages and bread through the controlled use of fermentation.

Alcoholic Beverages

In fermentation, starch is converted to simple sugars, such as sucrose and glucose, and through a complex sequence of some 12 reactions, these sugars then are converted to ethyl alcohol (the kind of alcohol that can be consumed, as opposed to methyl alcohol and other toxic forms) and carbon dioxide. Numerous enzymes are needed to carry out this sequence of reactions, the most important being zymase, which is found in yeast cells. These enzymes are sensitive to environmental conditions, such that when the concentration of alcohol reaches about 14%, they are deactivated. For this reason, no fermentation product (such as wine) can have an alcoholic concentration of more than about 14%. Stronger alcoholic beverages, such as whisky, are the result of another process, distillation.

The alcoholic beverages that can be produced by fermentation vary widely, depending primarily on two factors: the plant that is fermented and the enzymes used for fermentation. Depending on the materials available to them, various peoples have used grapes, berries, corn, rice, wheat, honey, potatoes, barley, hops, cactus juice, cassava roots, and other plant materials for fermentation to produce wines, beers, and other fermented drinks. The natural product used in making the beverage usually determines the name of the synthetic product. Thus, for instance, wine made with rice—a time-honored tradition in Japan—is known as sake, while a fermented beverage made from barley, hops, or malt sugar has a name very familiar to Americans: beer. Grapes make wine, but "wine" made from honey is known as mead.

Other Foods

Of course, ethyl alcohol is not the only useful product of fermentation or even of fermentation using yeast; so, too, are baked goods, such as bread. The carbon dioxide generated during fermentation is an important component of such items. When the batter for bread is mixed, a small amount of sugar and yeast is added. The bread then rises, which is more than just a figure of speech: it actually puffs up as a result of the fermentation of the sugar by enzymes in the yeast, which brings about the formation of carbon dioxide gas. The carbon dioxide gives the batter bulkiness and texture that would be lacking without the fermentation process. Another food-related application of fermentation is the production of one processed type of food from a raw, natural variety. The conversion of raw olives to the olives sold in stores, of cucumbers to pickles, and of cabbage to sauerkraut utilizes a particular bacterium that assists in a type of fermentation.

Industrial Applications

There is even ongoing research into the creation of edible products from the fermentation of petroleum. While this may seem a bit far-fetched, it is less difficult to comprehend powering cars with an environmentally friendly product of fermentation known as gasohol. Gasohol first started to make headlines in the 1970s, when an oil embargo and resulting increases in gas prices, combined with growing environmental concerns, raised the need for a type of fuel that would use less petroleum. A mixture of about 90% gasoline and 10% alcohol, gasohol burns more cleanly that gasoline alone and provides a promising method for using renewable resources (plant material) to extend the availability of a nonrenewable resource (petroleum). Furthermore, the alcohol needed for this product can be obtained from the fermentation of agricultural and municipal wastes.

The applications of fermentation span a wide spectrum, from medicines that go into people's bodies to the cleaning of waters containing human waste. Some antibiotics and other drugs are prepared by fermentation: for example, cortisone, used in treating arthritis, can be made by fermenting a plant steroid known as diosgenin. In the treatment of wastewater, anaerobic, or non-oxygen-dependent, bacteria are used to ferment organic material. Thus, solid wastes are converted to carbon dioxide, water, and mineral salts.

Where to Learn More

Asimov, Isaac. The Chemicals of Life: Enzymes, Vitamins, Hormones. New York: Abelard-Schulman, 1954.

"Enzymes: Classification, Structure, Mechanism." Washington State University Department of Chemistry (Web site). <http://www.chem.wsu.edu/Chem102/102-EnzStrClassMech.html>.

"Enzymes." HordeNet: Hardy Research Group, Department of Chemistry, The University of Akron (Web site). <http://ull.chemistry.uakron.edu/genobc/Chapter_20/>.

Fruton, Joseph S. A Skeptical Biochemist. Cambridge, MA: Harvard University Press, 1992.

"Introduction to Enzymes." Worthington Biochemical Corporation (Web site). <http://www.worthingtonbiochem.com/introBiochem/introEnzymes.html>.

Kornberg, Arthur. For the Love of Enzymes: The Odyssey of a Biochemist. Cambridge, MA: Harvard University Press, 1989.

"Milk Makes Me Sick: Exploration of the Basis of Lactose Intolerance." Exploratorium: The Museum of Science, Art, and Human Perception (Web site). <http://www.exploratorium.edu/snacks/milk_makes-me_sick/>.

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A catalytic protein produced by living cells. The chemical reactions involved in the digestion of foods, the biosynthesis of macromolecules, the controlled release and utilization of chemical energy, and other processes characteristic of life are all catalyzed by enzymes. In the absence of enzymes, these reactions would not take place at a significant rate. Several hundred different reactions can proceed simultaneously within a living cell, and the cell contains a comparable number of individual enzymes, each of which controls the rate of one or more of these reactions. The potentiality of a cell for growing, dividing, and performing specialized functions, such as contraction or transmission of nerve impulses, is determined by the complement of enzymes it possesses. Some representative enzymes, their sources, and reaction specificities are shown in the table.

Some representative enzymes, their sources, and reaction specificities

Enzyme	Some sources	Reaction catalyzed
Pepsin	Gastric juice	Hydrolysis of proteins to peptides and amino acids
Urease	Jackbean, bacteria	Hydrolysis of urea to ammonia and carbon dioxide
Amylase	Saliva, pancreatic juice	Hydrolysis of starch to maltose
Phosphorylase	Muscle, liver, plants	Reversible phosphorolysis of starch or glycogen to glucose-1-phosphate
Transaminases	Many animal and plant tissues	Transfer of an amino group from an amino acid to a keto acid
Phosphohexose isomerase	Muscle, yeast	Interconversion of glucose-6-phosphate and fructose-6-phosphate
Pyruvic carboxylase	Yeast, bacteria, plants	Decarboxylation of pyruvate to acetaldehyde and carbon dioxide
Catalase	Erythrocytes, liver	Decomposition of hydrogen peroxide to oxygen and water
Alcohol dehydrogenase	Liver	Oxidation of ethanol to acetaldehyde
Xanthine oxidase	Milk, liver	Oxidation of xanthine and hypoxanthine to uric acid

Characteristics

Enzymes can be isolated and are active outside the living cell. They are such efficient catalysts that they accelerate chemical reactions measurably, even at concentrations so low that they cannot be detected by most chemical tests for protein. Like other chemical reactions, enzyme-catalyzed reactions proceed only when accompanied by a decrease in free energy; at equilibrium the concentrations of reactants and products are the same in the presence of an enzyme as in its absence. An enzyme can catalyze an indefinite amount of chemical change without itself being diminished or altered by the reaction. However, because most isolated enzymes are relatively unstable, they often gradually lose activity under the conditions employed for their study.

Chemical nature

All enzymes are proteins. Their molecular weights range from about 10,000 to more than 1,000,000. Like other proteins, enzymes consist of chains of amino acids linked together by peptide bonds. An enzyme molecule may contain one or more of these polypeptide chains. The sequence of amino acids within the polypeptide chains is characteristic for each enzyme and is believed to determine the unique three-dimensional conformation in which the chains are folded. This conformation, which is necessary for the activity of the enzyme, is stabilized by interactions of amino acids in different parts of the peptide chains with each other and with the surrounding medium. These interactions are relatively weak and may be disrupted readily by high temperatures, acid or alkaline conditions, or changes in the polarity of the medium. Such changes lead to an unfolding of the peptide chains (denaturation) and a concomitant loss of enzymatic activity, solubility, and other properties characteristic of the native enzyme. Enzyme denaturation is sometimes reversible. See also Amino acids; Protein.

Many enzymes contain an additional, nonprotein component, termed a coenzyme or prosthetic group. This may be an organic molecule, often a vitamin derivative, or a metal ion. The coenzyme, in most instances, participates directly in the catalytic reaction. For example, it may serve as an intermediate carrier of a group being transferred from one substrate to another. Some enzymes have coenzymes that are tightly bound to the protein and difficult to remove, while others have coenzymes that dissociate readily. When the protein moiety (the apoenzyme) and the coenzyme are separated from each other, neither possesses the catalytic properties of the original conjugated protein (the holoenzyme). By simply mixing the apoenzyme and the coenzyme together, the fully active holoenzyme can often be reconstituted. The same coenzyme may be associated with many enzymes which catalyze different reactions. It is thus primarily the nature of the apoenzyme rather than that of the coenzyme which determines the specificity of the reaction. See also Coenzyme.

The complete amino acid sequence of several enzymes has been determined by chemical methods. By x-ray crystallographic methods even the exact three-dimensional molecular structure of a few enzymes has been deduced. See also X-ray crystallography.

Classification and nomenclature

Enzymes are usually classified and named according to the reaction they catalyze. The principal classes are as follows.

Oxidoreductases catalyze reactions involving electron transfer, and play an important role in cellular respiration and energy production. Some of them participate in the process of oxidative phosphorylation, whereby the energy released by the oxidation of carbohydrates and fats is utilized for the synthesis of adenosine triphosphate (ATP) and thus made directly available for energy-requiring reactions.

Transferases catalyze the transfer of a particular chemical group from one substance to another. Thus, transaminases transfer amino groups, transmethylases transfer methyl groups, and so on. An important subclass of this group are the kinases, which catalyze the phosphorylation of their substrates by transferring a phosphate group, usually from ATP, thereby activating an otherwise metabolically inert compound for further transformations.

Hydrolases catalyze the hydrolysis of proteins (proteinases and peptidases), nucleic acids (nucleases), starch (amylases), fats (lipases), phosphate esters (phosphatases), and other substances. Many hydrolases are secreted by the stomach, pancreas, and intestine and are responsible for the digestion of foods. Others participate in more specialized cellular functions. For example, cholinesterase, which catalyzes the hydrolysis of acetylcholine, plays an important role in the transmission of nervous impulses. See also Acetylcholine.

Lyases catalyze the nonhydrolytic cleavage of their substrate with the formation of a double bond. Examples are decarboxylases, which remove carboxyl groups as carbon dioxide, and dehydrases, which remove a molecule of water. The reverse reactions are catalyzed by the same enzymes.

Isomerases catalyze the interconversion of isomeric compounds.

Ligases, or synthetases, catalyze endergonic syntheses coupled with the exergonic hydrolysis of ATP. They allow the chemical energy stored in ATP to be utilized for driving reactions uphill.

Specificity

The majority of enzymes catalyze only one type of reaction and act on only one compound or on a group of closely related compounds. There must exist between an enzyme and its substrate a close fit, or complementarity. In many cases, a small structural change, even in a part of the molecule remote from that altered by the enzymatic reaction, abolishes the ability of a compound to serve as a substrate. An example of an enzyme highly specific for a single substrate is urease, which catalyzes the hydrolysis of urea to carbon dioxide and ammonia. On the other hand, some enzymes exhibit a less restricted specificity and act on a number of different compounds that possess a particular chemical group. This is termed group specificity.

A remarkable property of many enzymes is their high degree of stereospecificity, that is, their ability to discriminate between asymmetric molecules of the right-handed and left-handed configurations. An example of a stereospecific enzyme is L-amino acid oxidase. This enzyme catalyzes the oxidation of a variety of amino acids of the type R—CH(NH₂)COOH. The rate of oxidation varies greatly, depending on the nature of the R group, but only amino acids of the L configuration react. See also Stereochemistry.

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A protein that catalyses a metabolic reaction, so increasing its rate. Enzymes are specific for both the compounds acted on (the substrates) and the reactions carried out. Because of this, enzymes extracted from plants, animals, or micro-organisms, or those produced by genetic manipulation are widely used in the chemical, pharmaceutical, and food industries (e.g. chymosin in cheese making, maltase in beer production, for synthesis of vitamin C and citric acid).

Because they are proteins, enzymes are permanently inactivated by heat, strong acid or alkali, and other conditions which cause denaturation of proteins.

Many enzymes contain non-protein components which are essential for their function. These are known as prosthetic groups, coenzymes, or cofactors, and may be metal ions, metal ions in organic combination (e.g. haem in haemoglobin and cytochromes) or a variety of organic compounds, many of which are derived from vitamins. The (inactive) protein without its prosthetic group is known as the apo-enzyme, and the active assembly of protein plus prosthetic group is the holo-enzyme. See also enzyme activation assays.

Enzymes are proteins which act as biological catalysts accelerating specific chemical reactions, such as the digestion of food. Without enzymes, these reactions often require very high temperatures and pressures. Although enzymes take part in the reactions, they are not chemically altered by them. Consequently they are not used up and are required in relatively small concentrations. The body varies the concentration of a particular enzyme to regulate a specific activity; generally, the higher the enzyme concentration, the greater the rate of reaction.

Enzymes sometimes require additional, non-protein components to function properly; these are called cofactors. Many minerals and vitamins function as cofactors or coenzymes; deficiencies result in inefficient enzyme activity and ill health.

Enzymes work most effectively within narrow ranges of temperature and pH. Deviations cause the enzyme to change shape (denaturation) and to become less effective; this happens if the body overheats as a result of physical exertion or when lactic acid produced by anaerobic respiration lowers the pH of body fluids.

Enzymes are most familiarly associated with digestion, as substances in the alimentary tract that are necessary for the breakdown of food into simpler stuffs that can be absorbed into the body proper. These are indeed important, but they are in a small minority among the vast population of the body's enzymes. They also differ from the majority in acting outside rather than inside the cells that make them.

All living cells are teeming with enzymes. The name comes from the Greek meaning ‘in leaven’ or yeast. They are proteins, synthesized in cells, which act as catalysts, causing all the body's chemical processes to advance with the necessary rapidity and completeness. Enzymes are ubiquitous in body cells and fluids, and they are specific — each enzyme is responsible for catalyzing one particular chemical process. Their existence and their function came to be recognized during the nineteenth century; understanding advanced with burgeoning twentieth-century biochemistry; and molecular biologists continue to elucidate their ultimate structure and mode of action, and the genes that make them.

The names and nature of enzymes

The naming of enzymes in most cases reveals their function; ‘-ase’ is added to the name either of the substance (the substrate) on which they act (like peptidase for those acting on peptides), or of the type of reaction induced (such as hydrolase, for those causing hydrolysis, the splitting of a substance with addition of water, or transferase, for those moving some chemical group from one molecule to another). Some of the first enzymes to be discovered have unique names, such as pepsin in the stomach, and trypsin from the pancreas, which are both proteinases.

So what sort of proteins are they, and how do they function? With molecular masses of 10 000 to 1 000 000, enzymes are themselves large molecules, but some also exist in larger complexes that facilitate a sequence of changes. An enzyme molecule is a ‘globular’ protein that has an area on its surface to which can be bound only the specific substrate that the enzyme is designed to accept. This binding leads to changes in both molecules that result in the formation of the required product, and restoration of the enzyme molecule to its original state, ready to take on another substrate molecule. With progressively higher concentrations of substrate the rate of product yield increases, but the increment in rate diminishes as it approaches a maximum at a certain substrate concentration; beyond this point only an increase in the concentration of the enzyme itself can accelerate the process. This behaviour is consistent with progressive occupation of binding sites on all available enzymes, until they are all functioning at a maximal turnover rate.

Range and sites of enzyme function

Enzymes operate at every stage of life. Even the head of the sperm releases an enzyme that dissolves its path through the outer covering of the ovum to reach and penetrate it. Cell division in the embryo and throughout life involves replication of the DNA that carries the genetic information. A series of specific enzymes is needed for this, to unwind the double helix, to replicate it by the synthesis of new strands, and to put it and the new pairs back together again — whilst other enzymes meanwhile supply energy by the breakdown of adenosine triphosphate (ATP). Yet others are involved in the formation of messenger RNA and in all subsequent synthesis of proteins in a cell that results from the genetic coding.

Enzymes implement every event in the internal life of every cell in the body, and in its interaction with its environment. Each enzyme, or chain of enzymes acting in rapid sequence, has a specific function. There are those that are necessary for respiration and energy production; for transport mechanisms across the cell membrane and between internal components; for modifications of cellular metabolism in response to hormones; and for any specialized activity, including secretion by glandular cells, contraction by muscle cells, synthesis, release, and reuptake of neurotransmitters by nerve cells. The continual potential damage to tissues by the generation of free radicals is crucially limited by the body's antioxidant enzymes.

All cells have enzymes in their membrane, in the cytoplasm, and in the organelles within them. Those at the heart of cellular metabolism are the complex sequence of respiratory enzymes in the mitochondria that make possible the utilization of oxygen for the conversion of nutrient substrates to carbon dioxide and water, synthesis of ATP, and its breakdown for release of energy.

Cell membranes are furnished with ‘sodium pumps’ — protein molecules spanning the cell membrane that pump sodium ions out and potassium ions in. Facing inwards is an enzyme site that binds and breaks down ATP to supply the energy for pumping. Other enzyme molecules in the cell membrane may have, in addition to a site for substrate-binding, another that acts as receptor for a ‘messenger’ that activates the catalytic process: for example, the insulin receptor spans the cell membrane of muscle or fat cells; its outer site binds insulin, and its inner site handles the first of a series of enzyme-catalyzed reactions inside the cell that result in the several effects of insulin.

At synapses between nerves, and at neuromuscular junctions, enzymes are present that break down redundant neurotransmitters, preventing persistence of their effects. An example is acetylcholinesterase, found in the synaptic clefts on motor end plates in skeletal muscle, which hydrolyses excess acetylcholine, the neurotransmitter released by the motor nerve terminals.

Within skeletal muscle fibres, the enzymes vary according to their type of metabolism: whether it is predominantly aerobic (utilizing oxygen: ‘slow’ or ‘red’ muscle) or anaerobic (‘fast’ or ‘pale’ muscle). The sequence of events leading from activation of a muscle fibre by neurotransmitter, to contraction by means of interaction between myosin and actin filaments, depends on enzymes at every stage.

Enzymes in the blood

In the circulating blood there are enzymes both inside the blood cells, and outside in the plasma. Blood cells, in common with all cells, have the necessary enzymes for membrane transport and energy production. White blood cells have respiratory enzymes for aerobic metabolism, and others suited to their particular functions. Red blood cells are without mitochondria and respire anaerobically, so have enzymes appropriate to anaerobic glycolysis. Important for their function in whole-body respiratory gas exchange, they contain carbonic anhydrase, which promotes the uptake from the tissues of carbon dioxide and its carriage in the blood as bicarbonate, by catalyzing its combination with water to form carbonic acid, and its release in the lungs by this reaction in reverse.

Some enzymes exist as pro-enzymes or zymogens; they require some molecular change to be triggered into their active forms. These include proteins in the plasma that are involved in blood clotting: prothrombin is synthesized in the liver, and becomes thrombin when clotting is activated, and plasminogen can come into action as plasmin, a clot-dissolving enzyme. In the stomach, pepsinogen is secreted, and activated into pepsin by the acid that is secreted at the same site.

Enzymes that are normally secreted only into the gut or inside cells may, in pathological conditions, appear in significant quantities in the plasma, so that their measurement may be clinically useful. Examples are digestive enzymes that leak into the blood in acute pancreatitis, and creatine kinase, an enzyme from muscle tissue, that can appear in skeletal muscle disorders or, along with other intracellular enzymes, after a coronary thrombosis resulting in breakdown of some of the cardiac muscle.

Conditions for enzyme activity

All enzymes need the right environment for effective function, notably an optimal acidity, which differs in accordance with the site at which a particular enzyme acts (for example, more acidic inside cells than outside, and, for digestive enzymes, acidic in the stomach and alkaline in the duodenum). Like any chemical reactions, the rate of those that are catalyzed by enzymes varies with temperature. Local heat generation, for example in exercising muscle, enhances all such reactions within it. Likewise, whole-body metabolic rate increases in fever and decreases in hypothermia, because of the effect on all enzyme-catalyzed reactions. Extremes of pH or temperature irreversibly abolish enzyme activity, and so also do some substances that bind to the active sites of particular enzymes. These include an organophosphate ‘nerve gas’ that blocks acetylcholinesterase (causing persistent accumulation of acetylcholine at neuromuscular junctions, and thus uncontrollable muscle contraction). Poisoning by cyanide is due to blocking an essential enzyme in mitochondria and so fatally preventing all tissue respiration.

Medical applications

It is possible to inhibit the action of an enzyme without destroying it, and this has important therapeutic implications. There are substances that compete with the natural substrate for binding to an enzyme by having a similar structure, and others that act on other components of the enzyme molecule, preventing its ability to catalyze. Acetylcholinesterase inhibition is again an example — though in this context useful and reversible — in the treatment of the condition of myasthenia gravis, when the receptors on muscles cells for acetylcholine are deficient; the similar molecular structure of neostigmine allows it to bind to the enzyme, preventing binding and breakdown of acetylcholine; this can then accumulate sufficiently to enhance neuromuscular transmission. Drugs are used similarly to reverse the neuromuscular blockade deliberately induced during general anaesthesia. A different and important medical application of enzyme inhibition is in the use of antibiotics that block enzymes in microorganisms that are essential for their life or growth.

There are also many necessary co-enzymes, or co-factors for enzymes — organic non-protein molecules, smaller than the enzymes themselves, which either enhance or are necessary for the enzyme's activity. These again are widespread throughout the body, and are of many different molecular structures. Some require for their synthesis small amounts of essential substances from the diet. This is the basis of the need for the vitamins of the B group — they provide components for co-enzymes which could not otherwise be made in the body. Ions of several metals are also essential as co-factors, as well as for incorporation in some enzyme molecules themselves.

— Sheila Jennett

See also alimentary system; cell; cell membrane; metabolism; respiration; transport.

A protein that acts as a biological catalyst, accelerating the rate of specific biochemical reactions. An enzyme is not used up or changed in the reaction, and the enzyme cannot force a reaction to occur between molecules that would not otherwise react. Enzymes are denatured with time, and by changes in pH and temperature. The concentration of specific enzymes involved in energy systems, is an important determinant of athletic ability.

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Enzymes are proteins that act as biological catalysts. They decrease the amount of energy needed (activation energy) to start a metabolic reaction. Without enzymes, you would not be able to harvest energy and nutrients from your food. As an example, lactose intolerance is an inability to produce lactase, the enzyme that breaks down milk sugar (lactose). While this condition is not life-threatening for adults, it can have severe consequences for infants and children.

Previous question:	What products are produced by fermentation?
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IN BRIEF: Something produced in plant and animal cells that causes a chemical change in other substances.

A special enzyme in the stomach is responsible for digesting food properly.

LearnThatWord.com is a free vocabulary and spelling program where you only pay for results!

(en-zeyem)

A protein molecule that helps other organic molecules enter into chemical reactions with one another but is itself unaffected by these reactions. In other words, enzymes act as catalysts for organic biochemical reactions.

An enzyme is a catalyst that causes a reaction but does not change itself. Enzymes are widely used in the food industry. Most enzymes are considered GRAS substances and are therefore not considered food additives. Furthermore, they are likely to be considered incidental additives because they react with foods on a cellular basis. There are many different types of enzymes that perform a host of functions that can develop flavor, improve flavor, or change the physical or chemical characteristics of a food product. Pectinases react with fruit pectins making the development of fruit juice concentrates more efficient and clarification easier. Corn syrup is an enzyme reduction of cornstarch to simpler sugars. Lipases convert fats. For example, in cheeses, lipase enzymes are used to develop fatty acids from base fat systems. In an EMC, enzymes are used to curtail an otherwise necessary aging process. Rennet is usually used, but as this is an extract of cow's stomachs, the prohibition of mixing dairy and meat makes most EMCs not kosher. Chymosin™ is a fermented non-animal rennet and therefore can be used in this application for kosher systems. Proteolytic enzymes are used to make tough meat more edible. They are also used on byproduct meat called meat digests. These digests are developed to yield nutritious and flavorful products and are used in animals' feeds and as enhancers for pet palatability. Enzyme activity usually takes place under optimum conditions of moisture, temperature, and pH. The temperatures needed for enzyme reaction are usually far less than for other more typical chemical reactions. Due to this fact, some chemicals are being developed at low temperatures via this method. Fungal enzymes work in lower pHs (approximately 4.5), while bacterial enzymes are effective at a higher pH. Some enzymes require calcium and other metallic salts to be present for activation. Chelating agents like EDTA (ethylene diamine tetra acetate) can tie up these salts, inhibiting enzyme reaction. Immobilized enzymes are those that are bound to an insoluble GRAS medium. One such enzyme is the insoluble glucose isomerase enzyme, CFR 184.1372. See Catalyst, Ficin, Rennet, Microbial Protease, Amylase.

Common enzyme processes follow:

• Oxidation - Reduction of enzymes.
  
• Sulfhydryl oxidase (SH oxidase) - Links mercapto groups as in gluten molecules to achieve better binding (i.e., retards softening in breads).
  
• Glucose oxidase catalase - Oxidizes glucose to gluconic acid. Decreases time needed for dough to develop and improves dough consistency. Obtained from aspergillus niger.
  
• Lipoxygenase - Oxidizes unsaturated fats to corresponding aldehydes and acids.
  
• Ethanol dehydrogenase and alcohol oxidase - Oxidation of primary and secondary alcohols into aldehydes and ketones.
  
• Polyphenoxidase and lactase - Development of aromatic flavor and fragrance compounds. Polyphenoxidase is the enzyme responsible for the browning of fruits.
  
• Oxireductase - Glucose oxidase derived from Aspergillus niger, as is catalase. The latter is also derived from bovine liver or Micrococcus lysodeikticus.
  
• Isomerase - Glucose isomerase is derived by a number of different microorganisms such as Actinoplanes missouriensis, Bacillus coagulans, Streptomyces olivaceus, S. olivochromogenes, and S. rubiginosus.
  
• Protein Cross Linking.
  
• Transglutaminase - Cross links proteins for more stable doughs and larger volumes.
  
• Hydrolases - See separate listings for amylases, proteases, lipases, hemicellulases.
  
• Mixed carbohydrase and protease enzyme product - CFR 184.1027 derived from fermentation of B. licheniformis.
  
• Amylase and General Carbohydrases - A hydrolase that breaks down starch molecules. Coupled with glucoamylase and glucose isomerase, it is used in the development of high fructose corn syrup. Alpha amylase is used as a dough strengthener. Alpha amylase is derived from Aspergillus niger and oryzae, Rhizopus oryzae, Bacillus subtilis, barley malt, or Bacillus licheniformis. Beta amylase is derived from barley malt. Cellulase is derived from A. niger, and Trichoderma reesi. Beta glucanase is derived from A. niger, and B. subtilis.
  
• Glucoamylase (or Amyloglucosidase) - Derived from A. niger and A. oryzae, and Rhyzopus oryzae. Hemicellulase is derived from A. niger. Invertase is derived from Kluveromyces.
  
• Pectinase - Derived from A. niger or R. oryzae.
  
• Cellulase - Breaks the complex polysaccharide into simpler sugars. Used with pectinases to improve fruit juice yields. Fiber can be added after the processing of juices to improve the healthfulness of the juices.
  
• Pullulinase - Hydrolyzes specific sites of the glucosidic branch for easy hydrolyzation by glucoamylase to glucose.
  
• Hemicullulase - A hydrolase that breaks down hemicellulose into simpler sugars.
  
• Lipase - A hydrolase that breaks down fats into glycerine and fatty acids and mono and diglycerides. Used in many applications from development of EMCs, development of savory flavors through fat hydrolysis, production of mono and diglycerides as emulsifying agents in food systems, and in the modification of palm oil fat with the subsequent interesterification with stearic acid for use in confections like chocolates. Fractionated stearic or lauric fats can be used in chocolate-based products (compound coatings) where the heat sensitivity of fats in cocoa butter can be avoided. The enzyme is derived from three major sources: (1) The edible forestomach of kids, calves, and lambs. (2) Animal pancreas tissue. (3) Aspergillus spp. (oryzae or niger).
  
• Protease - A general hydrolase that breaks down proteins into simpler amino acids. Used to break down scrap muscle meat (meat digest) for use in pet foods as both a nutritional supplement with positive palatability enhancement. These enzymes are also used in the development of non-acid catalyzed HVPs and modified wheat gluten for more nutritional supplements for vegetarians. Bromelain is derived from pineapples (Ananas comusus and A. bracteatus). Ficin is derived from fig (Ficus spp.). Papain is derived from papaya (Carica Papaya). Pepsin is derived from porcine or other animal stomachs. Rennet is derived from the fourth stomach of ruminant animals, from Endothia parasitica or Mucor miehei and M. pusillus. Microbial proteases are available and are typically derived from cultures of aspergillus or bacillus species like A. niger, or A. oryzae, or Bacillus subtilis or B. licheniformis.
  
• Peptidase - Hydrolysis of casein and other proteins by attacking peptide linkages. Peptidase eliminates starchy taste by peptidolysis of oligopeptides in starch with peptidase derived from Lactococcus lactis.
  
• Endo and Exopeptidase - From Aspergillus oryzae, Flavorzyme™ and alkaline serine protease and Alcalase™. Able to hydrolyze many types of protein-based food products to develop enzyme-derived hydrolisates to replace HVPs, including gelatin, an otherwise difficult protein to hydrolyze. Also useful for the debittering of lactic acid by fermentation.
  
• Actinase - Removal of off flavors from old rice grains (hexanal and hexanol).
  
• Pectinase - Hydrolyzes pectins into simpler saccharides (sugars), increasing the yield of fruit juice.
  
• Lactase - Reduction of lactose in dairy products for people with specific lactose intolerance.
  

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or enzymatic by, of, involving, or relating to an enzyme or enzymes; catalysed by an enzyme or by enzymes.
—enzymically or enzymatically adv.

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(en′zīm)
n

A protein substance that acts as a catalyst to speed up metabolic and other processes involving organic materials. Some enzymes function within cells; others function in the extracellular fluids and tissue spaces and organs. They are active in all major tissue functions, such as cellular respiration, muscle contraction, digestive processes, and energy consumption, and are produced intracellularly.

For a list of words related to enzyme, see:

• Digestive and Excretory System - enzyme: complex protein substance produced by cells to catalyze metabolic processes such as digestion
• Physiology - enzyme: organic catalyst for physiological activity not used up in reaction
• Substances, Particles, and Atomic Architecture - enzyme: biological catalyst

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See words rhyming with "enzyme."

  See crossword solutions for the clue Enzyme.

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Dansk (Danish)
n. - enzym

Nederlands (Dutch)
enzym (versneller van biochemische processen)

Français (French)
n. - enzyme

Deutsch (German)
n. - Enzym

Ελληνική (Greek)
n. - (βιολ.) ένζυμο

Italiano (Italian)
enzima

Português (Portuguese)
n. - enzima (f) (Med.)

Русский (Russian)
энзим

Español (Spanish)
n. - enzima

Svenska (Swedish)
n. - enzym

中文（简体）(Chinese (Simplified))
酵素

中文（繁體）(Chinese (Traditional))
n. - 酵素

한국어 (Korean)
n. - 효소

日本語 (Japanese)
n. - 酵素

العربيه (Arabic)
‏(الاسم) خميرة, إنزين‏

עברית (Hebrew)
n. - ‮חלבון הפועל כזרז בריאקציה כימית מסוימת, חומר מתסיס, אנזים‬

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